Yep. It seems that barrel over-torquing could be the culprit. But along with that, I suspect that the barrel threads are improperly cut. Seems that the barrel thread depth is shallow and perhaps they should be less coarse, i.e., they should have more threads per inch to withstand the magnum pressures.
If there is not enough mating surface, the barrel threads could be losing their holding power during the reaction after the gun is fired. This would allow the barrel to be dislodged from the frame.
I have owned my 340PD for about 10 years and I have shot many full power 180 grain .357 magnums and a steady diet of 158 grain .38 Special +P. I regularly practice with this gun and it is a handful but it's manageable once you make the decision to do it. I guess I got one of the good ones. I literally tried to blow the gun up before I would trust it. I have used only factory ammo in this gun.
The gun is definitely a compromise but I bought it for its light weight because many times it's the only gun that I can effectively conceal. For all its compromises, the 340PD allows me to go armed when I otherwise might not be able.
So, to answer your question "are scandium/aluminum alloys really a good idea?", I would have to say that they are a good idea. They are not for everyone but if you want deep concealment and lightest weight in a respectable caliber, it's the best thing going.
